This invention relates to an optical transmission system with an improved dispersion map and corresponding method. The transmission system and method are particularly applicable to long haul submarine transmission systems.
Nonlinear optical effects such as four-wave mixing (FWM) and Cross-Phase Modulation (XPM) can degrade the optical signal transmission through long-haul optical networks. Increasing the dispersion in the fibers decreases both FWM and XPM. Dispersion causes broadening in transmitted optical pulses due to the difference in transmission speeds of light at different wavelengths. Because the pulse is broadened, the power density is decreased over the pulse, and thus optical effects which are non-linear in power density are reduced.
Dispersion units are typically given as picoseconds/nanometer-kilometer (ps/nm-km), where the kilometer units correspond to the length of the fiber. The dispersion product of a span of fiber is a measure of the dispersion accumulated over the span. The dispersion product for a fiber of length L with a dispersion D is the product of L and D, i.e., Lxc2x7D. Thus, the dispersion product of a span of fiber having individual section of length Li and dispersion Di is the sum of the individual dispersion products xcexa3 Lixc2x7Di. 
While dispersion reduces nonlinear effects such as FWM and XPM, the accumulated dispersion in these long-haul systems must be compensated. In long-haul repeatered transmission systems using optical fibers, the interplay of the accumulation of large amounts of the chromatic dispersion and self-phase modulation (SPM), creates noise and distortion in the optical system. Dispersion maps, i.e., the dispersion as a function of the transmission distance, attempt to minimize the effects of chromatic dispersion.
Current submarine transmission systems generally have span lengths in the 45-50 km range and use a dispersion map which provides an average dispersion at a wavelength of 1560 nm that is approximately xe2x88x922 ps/nm-km in the approximately 90% of the transmission spans. The negative dispersion fibers used in those spans may be single fiber types or combinations of two fibers, in which case the fiber following the amplifier has a larger effective area to reduce nonlinear effects and the second fiber has a lower dispersion slope. The dispersion slope of a fiber is the change in the dispersion per unit wavelength. After approximately 10 spans, the accumulated negative dispersion is then compensated at a given wavelength by an additional span of single mode fiber (SMF). We denote the combination of the approximately 10 negative dispersion spans with the compensating span of SMF as a block.
FIG. 1 shows the accumulated dispersion at the end channels for a 64 channel system with end channels having wavelengths of 1535 nm and 1561 nm. The dispersion map in FIG. 1 has a period of 520 km, which is compatible with typical distances of recirculating loop test-beds used to characterize the performance of the fibers. In this system the accumulated dispersion at a wavelength between the end channels is brought back to zero after nine spans of negative dispersion fiber. However, the accumulated dispersion is not compensated at other wavelengths.
The primary drawback to the dispersion map for the system of FIG. 1 is that the transmission fibers all have positive dispersion slope, which leads to the rapid accumulation of large dispersion differences across the channel plan. In other words, the accumulated dispersion varies substantially over the channels which have different corresponding wavelengths. The dispersion is kept close to zero only at the wavelength, between the end channels, that is compensated by the periodic insertion of SMF. The difference between the accumulated dispersion of the end channels in FIG. 1 is over 2000 ps/nm after 1040 km. For a typical submarine transmission distance of 6000 km, the accumulated dispersion or dispersion product would be over 10,000 ps/nm. This can be partially mitigated through the use of pre-compensation and post-compensation fibers. However, the propagation of more channels or over longer distances is prohibitively difficult with this dispersion map.
An advantage can be achieved if the fibers in an optical transmission system can be arranged to yield an average chromatic dispersion near zero for all the channels having wavelengths within the transmission wavelength range. However due to XPM effects, the average dispersion of each channel should be substantially different from zero in each transmission span, but should be periodically compensated to limit the accumulated dispersion. This dispersion compensation scheme should result in an increased number of channels over long distances for transmission applications which use a number of channels for transmission, such as wavelength division multiplexing (WDM). There are also significant cost savings in the elimination of most of the dispersion compensation at the transmitter and receiver ends of the transmission system.
One embodiment according to the present invention is an optical transmission system. The optical transmission system includes optical fiber and transmits in a predetermined wavelength range bounded by a first wavelength and a second wavelength and having a substantially central wavelength. The system comprises a series of consecutive blocks of optical fiber, the consecutive blocks optically coupled to each other. Each consecutive block of optical fiber comprises a first series of N spans of optical fiber optically coupled to each other, where Nxe2x89xa7zero, the spans of the first series arranged consecutively; a second series of M spans of optical fiber optically coupled to each other, where Mxe2x89xa7zero, the spans of the second series arranged consecutively; and a third series of O spans of optical fiber optically coupled to each other, where Oxe2x89xa7zero and N+Oxe2x89xa72, the spans of the third series arranged consecutively, the first, second, and third series arranged consecutively. The individual dispersion products of the first, second, and third series are substantially not zero at each of the first, substantially central, and second wavelengths, and the sum of the dispersion products of the first, second, and third series is substantially zero at each of the first, substantially central, and second wavelengths. The optical transmission system may also include a plurality of optical amplifiers that are positioned before each span to amplify the optical signal.
It will be understood, that fiber spans in addition to those forming the spans of the series can be configured to shape the pulses or compensate the optical signal dispersion. For example, a first set of fiber spans may be placed near the transmitter to broaden the optical pulses, thereby reducing both intra-channel and inter-channel non-linear effects. In an advantageous embodiment, the effect of the first set of fiber spans can be removed or compensated by a second set of fiber spans placed near the receiver.
Another embodiment according to the present invention is an optical transmission system. The optical transmission system includes optical fiber and transmits in a predetermined wavelength range bounded by a first wavelength and a second wavelength and having a selected wavelength and a substantially central wavelength. The system comprises a series of consecutive blocks of optical fiber, the consecutive blocks optically coupled to each other. Each consecutive block of optical fiber comprises a first series of N spans of optical fiber optically coupled to each other, where Nxe2x89xa7zero, the spans of the first series arranged consecutively; a second series of M spans of optical fiber optically coupled to each other, where Mxe2x89xa7zero, the spans of the second series arranged consecutively; and a third series of O spans of optical fiber optically coupled to each other, where Oxe2x89xa7zero and N+Oxe2x89xa72, the spans of the third series arranged consecutively, the first, second, and third series arranged consecutively. The individual dispersion products of the first, second, and third series are substantially not zero at the substantially central wavelength, the sum of the dispersion products of the first, second, and third series is substantially zero at the selected wavelength, and the average dispersion slope in each block at the selected wavelength is between xe2x88x920.01 and 0.01 ps/nm2-km. The optical transmission system may also include a plurality of optical amplifiers that are positioned before each span to amplify the optical signal.
According to another aspect of the invention, methods of transmitting an optical signal between a transmitter and a receiver in a predetermined wavelength range bounded by a first wavelength and a second wavelength are provided corresponding to the above optical transmission systems.